Lithium has been chemically extracted with NO2BF4 in acetonitrile medium from LiCo1-yAlyO2 (y=0.1 and 0.25) and LiCo1-yMgyO2 (y=0.06 and 0.1). The products have been characterized by a Rietveld refinement of the X-ray diffraction data, wet chemical analyses for lithium and oxygen contents (where possible), and infrared spectroscopy for metallic conduction. While all the lithium could be extracted from LiCo0.94Mg0.06O2, only 80%, 70%, and 80% of the lithium could be extracted, respectively, from LiCo0.9Al0.1O2, LiCo0.75Al0.25O2, and LiCo0.9Mg0.1O2. The Li1-xCo0.9Al0.1O2 system maintains the initial O3-type structure for 0.45less than or equal to(1-x)less than or equal to1.0 and a two-phase mixture of O3+O3' phases for 0.2less than or equal to(1-x)<0.45, while the Li1-xCo0.75Al0.25O2 system maintains the initial O3-type structure for the entire range of 0.3less than or equal to(1-x)less than or equal to1.0. Similarly, Li1-xCo0.94Mg0.06O2 maintains the initial O3-type structure for 0.6less than or equal to(1-x)less than or equal to1.0 and a mixture of the O3- and P3-type phases for 0<(1-x)less than or equal to0.6, while Li1-xCo0.9Mg0.1O2 maintains the initial O3-type structure for the entire range of 0.2less than or equal to(1-x)less than or equal to1.0. The observed phase relationships are compared with those of the previously investigated Li1-xCo1-yMyO2 and Li1-xNi1-yMnyO2 systems, and the results are rationalized on the basis of electrostatic considerations. (C) 2004 Elsevier B.V. All rights reserved.